1. Field of the Invention
The invention relates to a Radio Frequency Identification (RFID) writer-reader and a method for selecting an RFID data carrier from a number of RFID data carriers detected by the RFID writer-reader.
2. Description of the Related Art
For identifying workpieces and other objects, Radio Frequency Identification (RFID) data carriers (“transponders” or “TAGs”) and the associated RFID writer-readers (“readers”) are used particularly in industrial automation arrangements. Passive RFID data carriers are usually used, which are supplied with energy from a radio-frequency field generated by the RFID writer-reader and which transmit information to the RFID writer-reader by modulation (“backscattering”) of the radio-frequency field. The RFID writer-reader therefore has to simultaneously both transmit, and receive the modulated signal where, despite the fact that the received signal is based on the radio-frequency signal emitted by the RFID writer-reader, the received signal component can be regarded as emitted by the RFID data carrier.
Particularly in the case of RFID systems that operate in the Ultra-High Frequency (UHF) range, a plurality of RFID data carriers can be detected by the RFID writer-reader simultaneously. Although this is actually desired for many applications, such as for detecting all goods in a basket of goods, in many applications it is desirable for only a single RFID data carrier to be detected, to be precise regularly the one closest to the RFID writer-reader. If nevertheless a plurality of RFID data carriers are detected simultaneously in such arrangements, after detection an attempt must be made to select, i.e., filter out, the closest of the RFID data carriers.
Numerous procedures for performing such a selection are known in the prior art. By way of example, the received field strength Received Signal Strength Indication (RSSI) is registered for all received RFID data carriers, where it is assumed that that RFID data carrier having the best received field strength is also the closest, and so it is selected. On account of reflections of the radio waves, overshoots and other effects, however, it is possible that the closest of the RFID data carriers is not actually also the one that is received with the best signal strength. Besides optimizations in the radio field, for example by electromagnetic shielding of adjacent regions and the like, an attempt is often made to filter out the incorrectly selected RFID data carriers, i.e., “false positive reads”, by a corresponding processing logic, such as by comparison of the serial numbers (transponder ID) with expected values. However, accurate knowledge of the expected values is required for this purpose, and so a downstream processing logic is not always able to filter out the transponders which, although technically correctly detected, nevertheless are not currently associated with the actual reading task.
Further methods for selecting the transponder consist, for example, in dynamically adapting the transmission power of the RFID writer-reader, such as by progressively increasing the transmission power proceeding from a minimal value until exactly one RFID data carrier is detected; this method is also known as “power ramping”. Another method consists in starting a multiplicity of successive detection processes, that RFID data carrier which is detected most often then being selected, for example. This procedure can, if appropriate, also be combined with the “power ramping” method described above.